The mod is just slightly higher on the GPU requirements and higher on CPU requirements depending on the level itself (Jungle Joyride being the main CPU killer). The game is awful at managing resources properly and the terrain streamer will start actually loading less terrain if you play several levels in sequence than from a cold boot. I mention this because your framerate tests will be really inconsistent if you played something like Rooftop Run first rather than playing one or two levels before and then Rooftop Run afterwards. The HUB world itself is definitely more demanding than regular Generations due to a bit too many polygons, so a consistent framerate drop there would indicate a GPU bottleneck. Mind you, back when I did that I had a 6870 though, which is definitely way lower spec-ed than what you're running. That said, the posted requirements seem to fit completely fine considering I ran it at decent frame rates with worse hardware. Sounds like over-heating to me and automatic downclocking, or something else is using CPU/GPU while you're playing.